In prior apparatus used for the rotary draw bending of pipe and tube, such as of use in automobile exhaust systems, heat exchangers and aircraft construction, a primary component is the bending head of the apparatus. The bending head comprises a rotary bend die, an opposing clamp die which clamps a section of the tube immediately preceding the section of the tube where the bend is to be formed, and a pressure die located directly behind the clamped section of the tube. As the tube is pulled around the rotary bend die, the pressure die moves substantially in unison with the tube while resisting the radial reaction force of the tube action on the pressure die. Thus, the pressure die and rotary bend die cause the tube to be squeezed therebetween during the bending operation.
Many variable factors such as the type of tube material, tube wall thickness, shape of tube section to be formed, the radius of the bend and the like contribute to the "springback" characteristics of the tube and need to be considered when tube bending with rotary draw bending machine is carried out. However, although commercially acceptable tubes are manufactured with apparatus hereinbefore described, there is a need for pipe bending means which are capable of producing bent tubes of consistent quality on a repeated basis.
There are known apparatus having means available which can detect and compensate for the tube's springback characteristics. These apparatus are usually used after the tube is bent and unloaded from the bender to measure actual tube geometry compared to the desired tube geometry, such that the springback factor can be calculated and used to set the corrective bend angles needed to obtain the desired specified shape. Although these apparatus are effective, in practice there may be many iterations needed, with a high number of tubes being used, before the corrective springback factor is determined which results in increased tube scrap.
On some benders there are means for bend correction while the tube remains on the bender. In U.S. Pat. No. 5,275,031, issued Jan. 4, 1994, to Whiteside, J. A. et al., the bend correction means comprises a pressure die for holding a tube, a bend die for providing a radius form about which the tube is bent, and a clamping die for engaging and bending the tube relative to the pressure die and about a portion of the radius die. Control means moves the clamping die between a bend angle position in which the tube is bent about the radius die to a predetermined angle relative to angle position in which the tube is bent about the radius die to a predetermined angle relative to the clamping die and a relieved position in which the clamping die is not bending the tube. Sensor means, movable conjointly with the radius die, engages the tube in a springback position upon rotation of the radius die toward the tube and sends a signal to the control means upon contact with the tube. The control means then determines a rebend angle to achieve the desired degree of bend in the tube and moves the clamping die means and radius die to the rebend angle to bend the tube to that angle and storing the rebend angle for bending subsequent pieces. However, by having the sensor move conjointly with the radius die and having the radius die rotate until the sensor makes contact with the tube results in inaccurate position readings. Due to the sensitive nature of sensors, it will detect the location of the tube once it comes into contact with the tube. However, the contact location may not result in an accurate calculation of the actual tube angle if the sensor does not meet the tube at a flush 90 degree angle. By reason that the sensor is mounted on the bend die which rotates in an arc to make contact with the tube, it may not be possible for the sensor to make a 90 degree contact with the tube for all angle of bends. There is also the possibility that depending on the amount of springback of the tube, the sensor may not be able to make contact with the tube because it is firmly attached to the bend die. Further, due to the bending process itself there may be situations where lubricants are used on the tube around the bend die. These lubricants have the potential of clogging up the area where the sensor is mounted in the bend die to render the sensor ineffective or inaccurate.
Yet further, U.S. Pat. No. 5,275,031 corrects each angle of the tube through trial and error. If the specific angle is 30 degrees and the actual angle 27 degrees, produced on a first bend the angle at that portion of the tube will be rebent to an angle of greater than 30 degrees so that the relaxed state of the tube at that position will measure 30 degrees. In consequence, of the strain hardening nature of the materials, once a tube is bent, it is difficult to make minor adjustments to the bend. This strain hardening aspect of materials results in a bend angle error that is not compensated for in the control means calculation and may result in a final tube shape that does not correspond to the specified tube shape.